Even though you can't sing, and for what I'm going to suggest you don't need to. If your trying to put a solo over a rhythm, go throught each measure one at a time and hum something over it. Then figure it out on the guitar. You don't necessarily need to know each note you hummed by pitch. Just by looking at the chord your playing over and a scale that would go with that chord, you should be able to figure out the idea of what you were humming. This will train you to the highest level of any musician, being able to pull the music out of your head and onto the guitar. Practice this with your routine, just think up any melody in your head and then try to figure it out on the guitar. Instead of fiddling around until you come up with something cool. Works yes, but you will be a far more effective guitarist if you can pull ideas clearly out of your head.